Earthquake Resistant Building

An earthquake is a sudden, rapid shaking of the earth surface caused by the breaking and shifting of rocks beneath. During an earthquake, ground motion occurs in a random fashion in all directions radiating from a point within earth crust, called epicentre. It causes vibrations of structures and induces inertia forces on them. As a result, the structure may collapse resulting in loss of property and lives. Earthquakes do not kill people, vulnerable buildings do so. Hence there is a need for designing earthquake-resistant buildings, especially in the earthquake-prone areas.

TYPES OF EARTHQUAKE

Depending upon the possible causes, the earthquakes may be classified as:

[1] NATURAL EARTHQUAKE

Natural earthquakes may be due to:

  • Active Faults
  • Movement of tectonic plates or
  • Volcanic eruptions.

In earth’s crust, there are some faults which are not yet settled. The displacement of rocks along faults cause the earthquake. Tectonic means large scale process affecting the structure of the earth crust. This process causes gradual movement of material within the crust of the earth. Sometimes it shakes the earth crust. A volcano is a mountain or hill having a crater through which lava, rock fragments, hot vapour and gas are or have been erupted from the earth’s crust. Occasionally the volcanoes become active and create an earthquake near the mountain crater.

[2] EARTHQUAKE DUE TO INDUCED ACTIVITIES

These are caused by vibrations induced by atomic explosions and collapse of ground due to faulty mining.

IMPROVING AN EARTHQUAKE RESISTANCE FOR SMALL BUILDINGS

The earthquake resistance of small buildings may be increased by taking some precautions and measures in site selections, building planning and constructions as explained below:

[1] SITE SELECTION

The building constructions should be avoided on:

  • Near unstable embankments
  • On the sloping ground with columns of different heights
  • Flood affected areas
  • On subsoil with marked discontinuity like a rock in some portion and soil in some portion.
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[2] BUILDING PLANNING

Symmetrical plans are safer compared to unsymmetrical. Hence go for square or rectangular plans rather than L, E, H, T shaped. Rectangular plans should not have length more than twice the width.

[3] FOUNDATIONS

Width of foundation should not be less than 750 mm for single storey building and not less than 900 mm for storeyed buildings. Depth of foundation should not be less than 1.0 m for soft soil and 0.45 m for rocky ground. Before the foundation is laid remove all loose materials including water from the trench and compact the bottom. After the foundation is laid back-fill the foundation properly and compact.

[4] MASONRY

In case of stone masonry:

  • Place each stone flat on its broadest face.
  • Place length of stones into the thickness of wall to ensure interlocking inside and outside faces of the wall.
  • Fill the voids using small chips of the stones with minimum possible mortar.
  • Brak the stone to make it angular so that it has no rounded face.
  • At every 600 to 750 mm distance use through stones.

In case of brick masonry:

  • Use properly burnt bricks only.
  • Place bricks with its groove mark facing up to ensure better bond with next course.
  • Place bricks with its groove mark facing up to ensure better bond with next course.

In case of concrete blocks:

  • Place rough faces towards top and bottom to get good bond.
  • Blocks should be strong.
  • Brush the top and bottom faces before laying.

In general walls of more than 450 mm should be avoided. Length of wall should be Restricted to 6 m. Cross walls make the masonry stronger. It is better to build partition Walls along main walls interlinking the two.

[5] DOOR AND WINDOW OPENINGS

  • Walls with too many doors and windows close to each other collapse early.
  • Windows should be kept at same level.
  • The total width of all openings in wall should not exceed ⅓ rd the length of wall.
  • Doors should not be placed at the end of the wall. They should be at least at 500 mm from the cross wall.
  • Clear width between two openings should not be less than 600 mm.
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[6] ROOF

  • In sloping roofs with a span greater than 6m use trusses instead of rafters.
  • Building with 4 sided sloping roof is stronger than the one with two-sided sloping, since gable walls collapse early.

[7] CHEJJAS

Restrict chejjas or balcony projections to 0.9 m. For larger projections use beams and Columns.

[8] PARAPET

Masonry parapet wall can collapse easily. It is better to build parapet with bricks up to 300 mm followed by iron railings.

[9] CONCRETE AND MORTAR

Use river sand for making mortar and concrete. It should be sieved to remove pebbles. Silt should be removed by holding it against the wind. Coarse aggregates of size more than 30 mm should not be used. Aggregates should be well graded and angular. Before adding water cement and aggregates should be dry mixed thoroughly.

[10] BANDS

The following R.C. bands should be provided:

  • Plinth band
  • Lintel band
  • Roof band
  • Gable band.

For making R.C. bands minimum thickness is 75 mm and at least two bars of 8 mm diameters are required. They should be tied with steel limbs of 6 mm diameter at 150 mm spacing. If wall size is large, diagonal and vertical bands also may be provided.

[11] RETROFITTING

Retrofitting means preparing a structure in a scientific manner so that all elements of a building act as an integral unit. It is generally the most economical and fastest way to achieve safety of the building. The following are some of the methods in retrofitting:

  • Anchor roof truss to walls with brackets.
  • Provide bracings at the level of purlins and bottom chord members of trusses.
  • Strengthen gable wall by inserting a sloping belt on the gable wall.
  • Strengthen corners with seismic belts.
  • Anchor floor joists to walls with brackets.
  • Improve storey connections by providing vertical reinforcement.
  • Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.
  • Encase wall openings with reinforcements.

 

IMPROVING AN EARTHQUAKE RESISTANCE FOR TALL BUILDINGS

Tall buildings are subjected to heavy horizontal forces due to inertia during an earthquake. Hence they need shear walls. A shear wall is an R.C.C. enclosure within the building built to take shear forces. It is usually built around the lift room. These shear walls must be provided evenly throughout the buildings in both directions as well as from bottom to top. Apart from providing shear walls, the following techniques are also used for making tall buildings earthquake resistant:

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[1] BASE ISOLATION

The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building, or at least greatly reduced. The concept of base isolation is explained through an example of building resting on roller. When the ground shakes, the roller freely roll but the building above does not move.

If the gap between the building and the vertical wall of the foundation pit is small, the vertical wall of the pit may hit the wall. Hence 100% frictionless rollers are not provided in practice. The building is rested on flexible pads that offer resistance against lateral movements. This reduces some effect of ground shaking to the building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings.

[2] SEISMIC DAMPERS

Another approach for controlling seismic damage in buildings is by installing seismic dampers in place of structural elements, such as diagonal braces. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building.

Types of seismic isolation bearings:

  • High-density rubber bearings
  • Laminated rubber bearings and
  • Friction pendulum bearings.

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